: Obesity is highly prevalent in the United States and is associated with coronary heart disease, hypertension, diabetes, and cancer. Many studies have attempted to promote weight loss by restricting the intake of energy. By this method, weight loss has been mostly modest and not well maintained, and patients have reported a struggle to achieve and maintain the low calorie goals. The aim of the proposed research is to investigate whether a dietary treatment which allows food ad libitum, but restricts fat intake to a certain goal will be more easily incorporated into lifestyle and will produce better long term weight losses compared to a "standard" treatment which has a calorie goal. Two dietary intervention strategies will be investigated: a low fat ad libitum diet; and a restricted calorie diet. The proposed study will also examine the relationship between weight loss and personal and family history of obesity and body fat distribution to provide more insight into why some individuals have greater difficulty in losing and maintaining weight than others. In the proposed study, 118 obese women will be recruited. The subjects will be randomly assigned to one of the two treatments. The effectiveness of the treatments will be analyzed by assessing changes in body weight and body composition, food intake, palatability of diets, satiety, quality of life, barriers to adherence, and exercise behavior at 6 month intervals for 18 months. The study hopes to accomplish two objectives: 1) to develop a dietary weight loss program that will be acceptable over a prolonged period and produce better long term weight losses; and 2) to provide a better understanding of why some individuals have more difficulty in controlling their weight than others. Obesity is highly prevalent in the United States and is associated with coronary heart disease, hypertension, diabetes, and cancer. Many studies have attempted to promote weight loss by restricting the intake of energy. By this method, weight loss has been mostly modest and not well maintained, and patients have reported a struggle to achieve and maintain the low calorie goals. The aim of the proposed research is to investigate whether a dietary treatment which allows food ad libitum, but restricts fat intake to a certain goal will be more easily incorporated into lifestyle and will produce better long term weight losses compared to a "standard" treatment which has a calorie goal. Two dietary intervention strategies will be investigated: a low fat ad libitum diet; and a restricted calorie diet. The proposed study will also examine the relationship between weight loss and personal and family history of obesity and body fat distribution to provide more insight into why some individuals have greater difficulty in losing and maintaining weight than others. In the proposed study, 118 obese women will be recruited. The subjects will be randomly assigned to one of the two treatments. The effectiveness of the treatments will be analyzed by assessing changes in body weight and body composition, food intake, palatability of diets, satiety, quality of life, barriers to adherence, and exercise behavior at 6 month intervals for 18 months. The study hopes to accomplish two objectives: 1) to develop a dietary weight loss program that will be acceptable over a prolonged period and produce better long term weight losses; and 2) to provide a better understanding of why some individuals have more difficulty in controlling their weight than others.